A Venusian day, strange to say, is longer than a Venusian year. How can such an apparent paradox be allowed? It's easy: Venus rotates on its axis once every 243 earth-days, but it revolves around the sun in 225 earth-days. How appallingly counter-intuitive! If I had any gift at all for thinking in the abstract, I'm sure that I wouldn't find the idea that a day can be longer than a year so disorienting. But I do; for me, the word "day" has a metaphorical as well as a scientific dimension: "days" are short, "years" are long. I'm consoled by thinking that my counterpart, a philosophically-handicapped Venusian visiting the earth, would find our speedy days and our disproportionately long year quite incredible. On Venus, the lazy old sun takes 116 days to rise in the west and set in the east (Venus, ever the contrarian, rotates, so to speak, "backwards"). So much for Ovid's swift horses; on Venus, they're all glue and molasses, their forward progress barely perceptible. There's a mythological corollary that might please the poets. Randy Jupiter would be happy on Venus: the venereal night is truly a megala nux.
I suppose that the example of Venus teaches us that days and years are not things but concepts. For me, this is a hard lesson. I recognize that it's earthocentric to measure the long Venusian day in terms of earth-days, but I can't seem to reason my way around doing so -- the only day that I've ever experienced is 24 hours long and the only year with which I'm familiar has 365 and a bit days. It's alarming to realize how short-sighted it is to assume that a day, a month, a year, tied in, as they are, to astronomical reality are natural, when, in fact, they're only artifacts of our earthly orbit.
Other measurements of time are less "natural" and therefore even more troublesome. How entirely arbitrary to divide the day into 24 parts! Since everything else is going metric, why shouldn't we divide the day into ten or a hundred parts -- decidays or centidays? And minutes and seconds -- why sixty of them, when decihours and centihours makes so much more sense? (The ancients, incidentally, struggled mightily with hours -- read all about it here.) And the week? No natural reason at all for the week. The Romans, as is well known, were weekless.
Venus, having no moons, has no months. Mars has two moons -- would a Martian calendar, if there were such a thing, have had months, or demi-months, or monthettes, or what?
When I was a boy, I was a diligent if uncritical reader of science fiction. In those days, it was not uncommon for entire fanciful Venusian civilizations to rise, peak, and fall and the course of a novel. But in the 1950s, almost no data was available to us about the mysterious planet. Now, I'm afraid, the glamor is long gone. The surface temperature of Venus averages 869F, the atmospheric pressure is 90 times that of the earth, the atmosphere is 97 per cent carbon dioxide and contains no oxygen, and the pretty swirling clouds are entirely composed of hot, concentrated sulfuric acid. It's hardly a place in which an advanced culture would be likely to thrive nor a place from which we would expect much in the way of sophisticated calendrical theory.